Now that we are connected to the PSTN it is important to take care of some security considerations. Re-INVITES are being processed under the loose route section. These re-INVITES are not being challenged for its credentials. To enhance the security add the script below to your loose_route section. If the request is sequential (has_totag()) it need to have a ROUTE header. If it does not have (checked by the function loose_route()) we will discard the request with an error type "404, Not Here". Check the file openser.chapter7-3 if you have any doubt.

if (has_totag()) {
		# sequential request withing a dialog should
		# take the path determined by record-routing
		if (loose_route()) {
	           #Check authentication of re-invites
	           if(method=="INVITE" && (!allow_trusted())) {	
                      if (!proxy_authorize("","subscriber")) {
	                   proxy_challenge("","1");
	                   exit;
	               } else if (!check_from()) {...